1. Field of the Invention
The present invention generally relates to components subjected to high operating temperatures, such as gas turbine engine components. More particularly, this invention relates to a coating and coating process for incorporating surface features on an air-cooled surface of a component for the purpose of promoting heat transfer from the component.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature properties of the engine components must correspondingly increase. While significant advances have been achieved through formulation of iron, nickel and cobalt-base superalloys, the high temperature properties of such alloys are often insufficient to withstand long exposures to operating temperatures within the turbine, combustor and augmentor sections of some high-performance gas turbine engines. As a result, forced air cooling is often employed, alone or in combination with an environmental coating or a thermal barrier coating (TBC) system that, respectively, environmentally or thermally protects the component surfaces. In a typical cooling scheme, air is drawn from the engine compressor and flowed through or directed at surfaces of a component. In a technique known as backside air flow, jets of air are directed to impinge surfaces of a component that are not directly exposed to the high temperature combustion gases, e.g., the backside of a component.
The performance of a turbine component is directly related to the ability to achieve a generally uniform surface temperature with a limited amount of cooling air. In terms of the heat transfer mechanism, the performance of a conventional impingement-cooled surface is a function of the flow mechanism on the surface, the wetted surface area, and the temperature difference between the fluid and the surface. To promote uniform convective cooling of a component surface, it is conventional to increase the surface heat transfer coefficient of the cooled surface by forming heat transfer enhancement features, such as protuberances or bumps referred to as turbulators, on the surfaces of the component that require cooling. The size, shape and placement of turbulators affect the heat transfer rate from a component surface, and therefore affect the extent to which the service temperature of a component is reduced. Turbulators have been formed during casting of components, as taught in commonly-assigned U.S. Pat. No. 5,353,865 to Adiutori et al. However, casting techniques are limited in their ability to form dense patterns of small turbulators, which are desirable for backside air cooling because a single air jet is then able to impinge multiple turbulators. Furthermore, casting cannot be used to add, repair or modify turbulators on a component already in service.
Another approach to forming turbulators is a brazing technique taught by commonly-assigned U.S. Pat. No. 6,484,669 to Hasz et al., in which metallic particles are brazed to an air-cooled surface. This technique is able to achieve a good heat transfer enhancement in view of the ability to use small particles placed close together. However, if one were to deposit an environmental coating on the cooled surface to protect the component from hot corrosion and oxidation, the spaces between the brazed particles become filled with the coating material, thereby reducing the surface area enhancement. This problem is avoided by a method taught by commonly-assigned U.S. Pat. No. 6,254,997 to Rettig et al., in which an environmental overlay coating of MCrAlY (where M is iron, cobalt or iron) is deposited by electric arc wire thermal spraying. According to Rettig et al., an electric arc spray process is capable of depositing a relatively rough coating, preferably an average surface roughness (Ra) of greater than about 500 microinches (about 13 micrometers), which promotes heat transfer from the coated surface.
While improvements in cooling efficiency have been achieved with the above techniques, further enhancements in processing and thermal efficiency would be desirable.